Songs from Beneath the Sea

Marine mammals may have little in common with college students. But according to new research, their communal feeding habits come pretty close.

Consider: It’s the night before finals. You’re in a packed Snell Library when someone shouts: “Pizza at the front desk!”

You know what happens next.

A team led by Purnima Ratilal-Makris, associate professor in Northeastern’s Department of Electrical and Computer Engineering, has mapped a similar “mass feeding frenzy,” as Ratilal-Makris puts it, involving more than eight highly protected species of whales and dolphins in the U.S. Gulf of Maine region. It is the first time researchers have observed predator and prey interactions in the wild over such a vast expanse, including specific species’ feeding behaviors. Understanding how the two relate could have important implications for conserving marine ecosystems.

The research was published Wednesday in the prestigious journal Nature. It is especially timely, given concerns about climate change, including how water temperature, pollution from land runoff, overfishing, and other alterations affect life in ocean environments.

Ratilal-Makris and her colleagues have developed innovative underwater acoustic sensing methods to access crucial information that policymakers and marine managers can use, she says, “to model ocean ecosystems and formulate regulations for effective management of human activities in the ocean.”

audio: Humpback Whale/pictured: Humpback Whale

Sing for your supper

Ratilal-Makris and her team have been at the forefront of oceanic monitoring for decades. In two earlier papers published in Science, they described a new technology for mapping fish, including the Atlantic herring many whale species favor, and the remarkable convergence at night of billions of herring into shoal formations, each the size of Manhattan, on the northern flank of George’s Bank, where the fish go to spawn.

Called ocean acoustic waveguide remote sensing, or OAWRS, the technology comprises a densely sampled hydrophone array—essentially a long insulated electrical wire with 160 underwater microphones attached to it. The system surveys an area of an astonishing 100,000 square kilometers and instantaneously returns images derived from projected sound scattered by the herring’s air-filled bladders back to the researchers aboard the ship.

audio: Minke Whale/pictured: Pilot WhalesIn the new study, Ratilal-Makris paired OAWRS with POAWRS, or “passive OAWRS,” over a two-week span. The technology uses the same hydrophone array to pick up and record calls of various marine mammals and then maps the animals’ locations and pinpoints their species.

POAWRS took almost six years to develop, says Ratilal-Makris. “We had to learn all the different sounds that marine mammals produce, what was unique to each species, and then create a methodology using the hydrophone array not only to detect the sounds but to locate what direction they were coming from and their distance from us,” she says. They also had to construct a “classifier” to match the vocalizations to the individual marine mammal species. Among those the system tracked: blue, fin, humpback, sei, minke, sperm, pilot, and killer whales and a variety of dolphins, each with its own unique call pattern.

The humpbacks’ vocalizations, lyrically, tipped into song. “As with human songs there are themes that repeat,” says Ratilal-Makris. “We recorded many of their repeated themes—some went on for hours.”

audio: Fin Whale/pictured: Pilot Whales

Sticking with their kind

A feeding frenzy may bring disparate groups together at once, circling their common “prey,” but that doesn’t mean they necessarily dine together. College students, for example, may descend upon that trove of pizzas en masse, but they generally do their eating in like-minded groups.

So it is with marine mammals, Ratilal-Makris and her colleagues discovered. The whales arranged themselves around the herring shoals, she explains, in “species-specific foraging centers with varying degrees of overlap.” That meant a “hotspot” for the humpback whales here, a “hotspot” for the minke whales there, yet another for the fin whales, and so on.

“Before this work, nobody knew how the different whale species organized in relation to their fish prey,” says Ratilal-Makris. “Our study shows that they are not all mixed evenly—there are really well-defined feeding centers for each species.” What does that mean? “Perhaps they want to stick to their own kind,” she says.

audio: Humpback Whale/pictured: Humpback WhaleIt’s a question she will explore in subsequent research. Another next step, says Ratilal-Makris, is to develop the means to make more precise “abundance estimates.” “We can identify the direction and location of the sounds of the various species, but so far we can’t estimate size of the populations very accurately,” she says.

Knowing the species’ whereabouts, as well as their growth and reduction in relation to environmental changes, would help scientists better identify, and respond to, sources of ecosystem imbalance.

“What the study provides is exciting new insight into a previously hidden world,” explained one of the paper’s academic reviewers. “I think this manuscript is ground-breaking.”

audio: Sei Whale/pictured: Pilot Whales

Research collaborators included the Massachusetts Institute of Technology, the National Oceanic and Atmospheric Administration, and the Institute of Marine Research in Norway, with sponsorship from the Office of Naval Research, the National Science Foundation, the National Oceanographic Partnership Program, and the Sloan Foundation.